The present invention relates to a method of obtaining an optical fiber preform by employing a plasma build-up step. It also covers the preform that results from the method and an optical fiber drawn from the preform.
The manufacture of optical fibers from preforms is conventional. Obtaining a preform by the plasma build-up technique is known; it is described, for example, in patent document EP-450 465 A1.
The opto-geometrical characteristics of a fiber determine its transmission characteristics. These include the shape of the fiber, and in particular the areas of the various concentric portions of the fiber in cross-section, and the optical characteristics of these various portions. Such opto-geometrical characteristics result from the corresponding characteristics of the preform from which the fiber is drawn, after taking account of the effects of fiber-drawing.
An ideal fiber would have nominal and constant opto-geometrical characteristic along its entire length. It would be drawn from a preform that was perfectly cylindrical and that had constant opto-geometrical characteristics along its entire working length.
Unfortunately, in practice, various effects that are difficult to correct in full prevent the above ideal from being achieved. Thus, patent document EP 440 130 A1 mentions not only defects of concentricity, to which no further attention is given herein, but also mentions irregularities in diameter. These are corrected by machining the final preform, thus requiring one or more correction stages, and therefore giving rise to extra cost and loss of efficiency in production. In addition, the correction method concerned relates to preforms obtained directly by the modified chemical vapor deposition (MCVD) method, and not to preforms obtained by plasma build-up.
An object of the present invention is thus to provide a method of obtaining an optical fiber preform using plasma build-up that provides a preform having improved opto-geometrical properties.
According to a first characteristic of the invention, this object is achieved in that at least one of the plasma build-up passes is modulated in order to improve the opto-geometrical properties of the preform.
The invention is thus based on the principle consisting in modifying the opto-geometrical properties of the preform by acting on the plasma build-up process, and as a result it requires no additional correction stage of step.
According to a characteristic of the invention, said modulated build-up pass is one of the last build-up passes during which, the build-up process being un-modulated, the thickness of the built-up layer as deposited is substantially constant along the entire working portion of the preform.
It has been observed that one of the causes of axial variation in the opto-geometrical properties of preforms obtained in application of known plasma build-up methods lies not only in the primary preform that is to be built up, made by any known method (MCVD, VAD, OVPD, . . . ), but also in the plasma build-up method itself, and specifically in the first plasma build-up passes. Thereafter, still in the known method, the thickness of the plasma build-up layers finally becomes constant. The invention thus provides for acting at this moment on the plasma build-up process so as to correct for any deviations in the opto-geometrical properties of the preform.
More particularly, according to the invention, the build-up process is modulated from a predetermined diameter of the preform.
In another aspect of the invention, the modulated build-up process consists in controlled axial modification of at least one of the plasma build-up parameters, and in particular the flow rate of build-up grains, the temperature of the plasma, or the axial speed of translation of the plasma along the preform. Preferably, the parameter used is build-up grain flow rate.
In addition, according to another aspect of the invention, said controlled axial modification includes evaluating axial errors in the diameter of the preform and axially modifying said parameter so as to cause said modulated build-up pass to reduce said axial errors in diameter. In other words, depending on previously determined axial errors in diameter, the build-up pass is modulated to add toxe2x80x94or remove fromxe2x80x94the diameter of the preform so that said errors are made small or even eliminated after the build-up pass.
More precisely, according to the invention, it is possible either:
to reduce the grain flow rate proportionally to said error so as to deposit a layer that is of thickness which decreases with increasing excess diameter;
to reduce the grain flow rate in determined manner as to deposit a layer of reduced thickness where the diameter is too great, thereby bringing it down to a determined value; or else
to shut off the flow of grains completely, thereby giving rise to localized removal of matter from the preform by evaporation under the effect of the plasma.
According to yet another aspect of the invention, said evaluation of axial error is performed by axial diameter measurement during a measurement pass, which pass may advantageously coincide with a build-up pass, said axial errors being determined by comparing the results of said measurement with a reference profile.
In a first implementation, said reference profile comprises the minimum axial diameter observed during said measurement pass. That corresponds to errors relative to a cylindrical shape. However, in a second implementation, said reference profile also comprises an axial correction term for the diameter derived from measurements performed on the preform prior to plasma build-up. This makes it possible to take account of and to correct anomalies in the opto-geometrical characteristics of the preform that stem from stages prior to plasma build-up, such as tapering or central thinning of the core of the preform relative to its outside diameter.
The present invention also provides a preform and a fiber drawn from such a preform, having a partial build-up layer structure that results from application of the method as described above for obtaining an optical fiber preform.